**Same issue in version 12.5**
Workarround:
- Try upgrade
admin:utils system upgrade initiate
Source:
1) Remote Filesystem via SFTP2) Remote Filesystem via FTP3) Local DVD/CD4) Local Image <None>q) quit
Please select an option (1 - 4 or "q" 3Please ...
Same failure.
It did not work to change port or DNS.
The test that was successful was to register to CUCM and then to Webex from CLI.
xCommand Webex Registration Start ActivationCode: <Webexcode> SecurityAction: NoAction